The objectives of this project are to study the assembly of proteins in the main filaments of skeletal muscle myofibrils and to describe the protein-protein interactions which are related to the calcium control of contraction and relaxation. Studies are being conducted on the ultrastructure of tropomyosin plus troponin subunit crystals. Different forms of tropomyosin have been isolated and their properties are under examination. Experiments are also in progress to determine the location of the troponin binding site on the tropomyosin molecule.